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Full text of "Text Book Of Mechanical Engineering"

8oo

Appendix II.

corrodes rather more than aluminium bronze, but is very close-
grained, and therefore suitable for resisting fluid pressure.

Aluminium is procured from its ores by one or other of two
processes. The older or Deville process has been much improved
as the Castner process, and is thus practised at Oldbury, near
Birmingham, being based on the displacement of aluminium from
its ores by metallic sodium. Caustic soda and iron carbide are
melted in furnaces at 1470° Fahr., and, thus being kept for some
i|- hours, the sodium distils over into iron condensers, and is
afterwards cast into blocks of 2 Ibs. each. Some 20 furnaces are
kept going at once, each producing 60 Ibs. of sodium per day,,
with an expenditure of 360 Ibs. caustic soda and 300 Ibs. iron
carbide.

Next, alumina is prepared, by mixing the ground mineral
(bauxite) with soda ash, and heating it in a furnace till silicate
and soda aluminates are formed, after washing which, first with
water and then with hydrochloric acid, the hydrate of alumina
remains. This is mixed with common salt and charcoal into a
paste, and made into balls, which are thoroughly dried and
heated in earthen cylinders. While in this condition perfectly-
dry chlorine gas is passed over them, and aluminium bichloride
distils over. .»

Lastly, 80 Ibs. of the bichloride, 25 Ibs. of metallic sodium,
and 30 Ibs. of cryolite (another ore of aluminium) as a flux, are
heated together to 1830° Fahr.; and metallic aluminium to the
weight of 8 Ibs. is thereby produced, impure only to the extent
of a %.

In the Cowles process the ore is directly reduced in electric
furnaces, or rectangular fireclay pits kept hot by the current from
an enormous dynamo. Each pole within the furnace consists of a
bundle of five 3-inch carbons, having metallic caps or heads—of
iron if ferro-aluminium is required, and of copper if for aluminium
bronze. The furnace lining is made of lime and charcoal powder,,
the latter for localising the heat and saving the furnace materials.
The charge consists of ore, metal (copper or iron as desired), -and
charcoal; and the resulting alloys contain 15 to 17% of aluminium.
Aluminium has only a third the specific gravity of iron, and is
practically untarnishable. The addition of \ to i % to cast iron-